As a method of reducing noise, ANC (active noise control) is known. ANC outputs a signal (control sound) having the same amplitude and an opposite phase as compared with noise from a control loudspeaker in order to reduce noise. As a basic technique for ANC, a technique called Filtered-x is known. ANC techniques are roughly classified into two types: a feedforward type and a feedback type.
An MRI (Magnetic Resonance Imaging) device generates very large repetitive impact noise because the device applies a slice selection gradient field for each TR (Repetition Time) in which an MR (Magnetic Resonance) signal is detected. Such impact noise has a high contribution ratio in noise generated by the MRI device. Since the MRI device is a strong magnetic field generator, a control loudspeaker that is a magnetic body cannot be arranged in the MRI device. In feedforward ANC, the distance between a control loudspeaker and an error microphone for evaluating a control effect needs to be shorter than that between a reference microphone for acquiring noise and the error microphone. For this reason, it is impossible to apply feedforward ANC to an MRI device in which a control loudspeaker cannot be arranged.
General feedback ANC is a technique of generating a control signal based on the latest detection signal, and hence can reduce only periodic noise in principle. When, therefore, feedback ANC is applied to an MRI device, the device can reduce periodic noise caused by the vibration of the gradient coil when performing phase encoding or reading, but cannot reduce impact noise generated for each TR.